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Aggregate shape and texture: strength, workability

Short answer

Smooth rounded aggregates improve workability and reduce water demand but give lower strength due to poor bond. Rough angular aggregates increase bond and strength (10–20% higher) but reduce workability and raise cement requirement. Flaky and elongated particles harm strength and durability; IS 383 limits combined flakiness and elongation index to 40%.

What are the effects of the shape and texture of aggregates on the strength, workability and economy of concrete?

Shape: particle shapes are classified as rounded, irregular, angular, flaky and elongated (IS 383).

Texture: the surface may be glassy, smooth, granular, rough, crystalline or honeycombed.

PropertySmooth, rounded aggregateRough, angular aggregate
WorkabilityHigher (less friction, less surface area)Lower; needs more water/paste
Bond with pasteWeaker (little mechanical interlock)Stronger bond (interlocking)
Compressive strengthLower, especially at low w/cHigher (10-20% more in high-strength concrete)
Flexural strengthLowerHigher (strength depends strongly on bond)
Water and cement demandLessMore for the same slump
EconomyCheaper to place, but gravel may cost more in hilly areasCrushed stone is costlier to produce, needs more cement and more compaction effort

Flaky and elongated particles reduce workability, strength and durability, since they lie in planes and hold bleed water under them. IS 383:2016 limits the combined flakiness and elongation index to 40 per cent (tested as per IS 2386 Part 1).

Economy: rounded aggregates reduce the paste requirement, but angular crushed aggregates give higher strength through better bond; the designer balances the extra cement against this benefit.

✓ Answer checked by Claude Opus 5.5 (high effort)

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